Background Image (Movie clip) appearing purple
Pink always mean that the image used is missing or cannot be displayed. 

If the frame you are trying to use does not exist in your video file you can instruct blender where to start the video file.

as described in this link:

How can I make a &quot;Movie Clip&quot; node start at a certain frame of the scene?

use the frame, start frame and offset for the video or image sequence.

 



Now why would your video be missing the first frame?

It is likely that you are dealing with video that has been compressed with an interframe codec using P or B frames and the reference keyframe at the beginning of the Group of Pictures is missing or cannot be read, so the first image displayed is the first keyframe of the next GOP. For more info on this read: https://www.bhphotovideo.com/explora/video/tips-and-solutions/compression-a-basic-understanding